This invention relates to an antenna, and in particular an antenna having two resonant frequencies.
Wireless portable communication devices typically have small antennas to allow ease of use of the communication devices. However, the size of the antenna typically results in the antenna having a narrow operational frequency band.
Some communication devices, however, are required to operate over different frequency ranges which differ significantly from one another. For example, a dual mode radiotelephone may be required to operate over more than one cellular telephone system. One such cellular system is Global System for Mobile Telecommunications (GSM) which operates over the 890 to 960 MHz frequency band while another cellular system that the radiotelephone may be required to operate on is the Personal Communication Network (PCN) which operates over the 1710 to 1880 MHz frequency band.
Antennas designed for radiotelephones are typically made of simple cylindrical coil or helical antennae or whip antennae made from a straight conductor. Where its electrical length, which should be a specific part of the wavelength of the radio frequency used, determines the resonant frequency of an antenna. For radiotelephones the size of the antenna is minimised by choosing an electrical length which is a fraction of the wavelength used. For a helical and whip antennae the electrical length should preferably be 3.lambda./8 or .lambda./4, where .lambda. is the wavelength. However, the characteristics of these antennae do not allow the radiotelephone to operate satisfactorily over two different frequency ranges which differ significantly from one another.
One solution to this problem has been to provide radiotelephones with two interchangeable antennae which have different resonant frequencies. Where the user attaches to the radiotelephone the antenna which corresponds to the frequency range of the system in use at any one time. However, continued exchange of the antenna can result in damage to the antenna connector and may cause contact disturbance. Further, a user could easily misplace the antenna currently not in use.
A second solution to this problem has been to attach, during manufacture, two separate antennae to the radiotelephone, each antenna designed to operate over different frequencies. Where the user or radiotelephone selects the antenna according to which system the radiotelephone is operating in. However, this solution increases the complexity of the radiotelephone design and thus increases the manufacturing costs of the radiotelephone.
European Patent 0 825 672 A1 describes an antenna which can operate at two separate frequencies. However, the antenna has three distinct specially designed elements which interact in a complex manner to provide the two frequencies. As such, this increases the complexity of the antenna design.
European Patent 0 593 185 A1 describes an antenna which has two separate resonating elements mounted within a common housing which allows the antenna to operate at two separate frequencies.